Ceramic thermal barrier coatings (TBCs) have received increased attention for advanced gas turbine engine applications. TBCs may be used to protect the components of a gas turbine engine that are subjected to extremely high temperatures. Typical TBCs include those formed of yttria stabilized zirconia (also referred to as yttria stabilized zirconium oxide) (YSZ) and ytrria stabilized hafnia (YSH). TBC systems have been aggressively designed for the thermal protection of engine hot section components, thus allowing significant increases in engine operating temperatures, fuel efficiency and reliability. However, the increases in engine temperature can raise considerable coating durability issues. The development of next generation lower thermal conductivity and improved thermal stability TBCs thus becomes important for advancing the ultra-efficient and low emission gas turbine engine technology.
An effective TBC has a low thermal conductivity and strongly adheres to the substrate to which it is bonded under use conditions. To promote adhesion and to extend the service life of a TBC, an oxidation-resistant bond coating is commonly employed. Bond coatings typically are in the form of overlay coatings such as MCrAlX, where M is a transition metal such as iron, cobalt, and/or nickel, and X is yttrium or another rare earth element. Bond coatings also can be diffusion coatings such as a simple aluminide of platinum aluminide. When a diffusion bond coating is applied to a substrate, a zone of interdiffusion forms between the bond coat and the substrate. During exposure of ceramic TBCs to high temperatures, such as during ordinary service use thereof, bond coats of the type described above oxidize to form a tightly adherent alumina scale that protects the underlying structure from catastrophic oxidation. The TBC is bonded to the bond coat by this alumina scale. The quality of the scale therefore is extremely important. During use, the alumina scale slowly oxidizes and grows in thickness at the extremely high use temperatures. This growth increases the stress on the TBC due to thermal expansion mismatch between the ceramic TBC and the metal substrate and the bond coat.
Partial loss of cohesion between a TBC and the underlying bond coating may contribute to TBC spalling. When this partial loss of cohesion occurs, alumina growth stresses and alumina-superalloy thermal expansion mismatch stresses within the thermally grown oxide, which occur during thermal transients, may form microbuckles in the thermally grown oxide at the TBC-bond coating interface. Once initiated, interfacial microbuckles continue to grow at operational temperatures in the range of 900 to 1150° C. because bond coatings have insufficient creep-strength to constrain the area-growth of the thermally grown oxide scale. The problem is compounded if the bond coating does not have an optimal chemistry or comprises impurities, such as sulfur or chlorine, that accelerate the oxidation of the bond coating and hence shorten the TBC life.
Accordingly, it is desirable to provide protective coating systems for gas turbine engine applications that exhibit long life and high reliability. It also is desirable to provide protective coating systems that have a low rate of oxidation and hence growth in thickness of the alumina scale so that thermal mismatch stresses do not increase during use. In addition, it is desirable to provide protective coating systems that minimize or eliminate TBC spalling. It is also desirable to provide methods for fabricating such protective coating systems. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.